1. Field of the Invention
This invention relates to ribozymes that cleave RNA, and more specifically to the enhancement of ribozyme catalytic activity using a facilitator oligonucleotide complementary to an RNA sequence contiguous to the ribozyme.
2. Description of the Related Art
Drugs might be based on RNA catalysts (ribozymes) designed to cleave viral or messenger RNA with high specificity at a rapid rate. These requirements historically have been mutually limiting.
Ribozymes consist of a catalytic core having flanking sequences adjacent the core which hybridize to the substrate RNA. The simplest catalytic core is an RNA motif known as a hammerhead.
Ribozyme specificity depends on the number of base pairs formed between the ribozyme flanking sequences and its RNA substrate. Increased base pairing has been shown to decrease the rate of cleavage. Goodchild and Kohli, Arch. Biochem. Biophys., 284: 386-391 (1991). Goodchild and Kohli studied the cleavage of a sequence from HIV-1RNA by various hammerhead ribozymes and determined that the rate of cleavage was dependent on the length of the flanking sequence. Shorter sequences were shown to result in weaker binding between the ribozyme and the cleavage products together with increased rate of cleavage. A ribozyme with 12 flanking sequences cleaved 10 times faster then one with 20 bases.
However, to have the requisite selectivity or specifity, i.e., the ability to discriminate between all RNA molecules in a cell, a ribozyme must form a minimum of about 15 base pairs with the target substrate. This requirement for selectivity limits the rate of cleavage that may be realized.
Accordingly, ribozymes having increased catalytic activity or methods of increasing ribozyme catalytic activity are needed.
Koizumi et al., FEBS Lett. 239: 285-288 (1988), discuss the design of two ribozymes for site-specific cleavage of RNA. A UA site in an undecaribonucleotide was cleaved by a ribozyme consisting of two partially paired oilgoribonucleotides with chain lengths of 19 and 15. The other ribozyme, which consists of 19-mer and a 13-mer, recognized a UC sequence at positions 42 and 43 of 5 S rRNA.
Haseloff and Gerlach, Nature 334: 585-59 (1988), discuss the dissection of the RNA substrate and enzyme activities from a single self-cleaving domain from the (+) strand of the satellite RNA of tobacco ringspot virus (sTobRV). Inspection of the separated substrate and ribozyme activities, in comparison with other naturally-occurring self-cleaving domains, led to a model for the design of oligoribonucleotides which posses new and highly sequence-specific endoribonuclease activities. This model was successfully tested by the design and construction of ribozymes targeted against three sites within the Tn9 chloramphenicol acetyl-transferase (CAT) messenger RNA sequence.
Hampel and Tritz, Biochemistry 28: 4929-4933 (1989), identified an RNA catalytic domain within the sequence of the 359 base long negative-strand satellite RNA of tobacco ringspot virus. The catalytic domain contains two minimal sequences of RNA, a 50 base catalytic RNA sequence, and a 14 base substrate RNA sequence. The catalytic complex of catalytic RNA/substrate RNA represents a structure not previously found in any RNA catalytic reaction.
Hampel et al., Nucleic Acids Res. 18: 299-304 (1990) discuss the identification of the catalytic domain within the sequence of the negative strand of the satellite RNA of tobacco ringspot virus. Minimum energy RNA folding calculations predict a two dimensional model with four major helical regions which are supported by mutagenesis experiments. This model for the catalytic complex consists of a 50 base catalytic RNA and a 14 base substrate RNA folded together in a type of hairpin two dimensional structure. Part of the recognition region between the catalyst and substrate is two helices of 6 bases and 4 bases respectively. Catalytic activity remains when the bases in these two helices are changed but base pairing is maintained. Thus an appropriately engineered `hairpin` catalyst is capable of cleaving heterologous RNA.
Uhlenbeck, Nature, 328: 596-600 (1987) describes the synthesis of two oligoribonucleotides that can combine to form a structure consistent with the consensus self-cleaving domain. Because rapid cleavage of one of the oligomers was observed only when the other was present, the domain was necessary and sufficient for cleavage. The properties of the cleavage reaction were studied in detail. Nearly complete cleavage occurred even with large excess of the oligomer that was cleaved. This indicates that the oligomer that is uncleaved can cycle in the reaction and therefore be considered to act as a catalyst in the cleavage of the other oligomer.
Fedor and Uhlenbeck, Proc. Natl. Acad. Sci. USA 87: 1668-1672 (1990), analyzed the kinetics of cleavage for several hammerhead sequences to characterize the reaction mechanism and explore how nucleotides involved in substrate binding affect cleavage.
Goodchild et al., Arch. Biochem. Biophys. 263: 401-409 (1988) discusses the effects of a series of synthetic oligonucleotides (hybridons) complementary to the 5' non-coding regions of rabbit .beta.-globin mRNA on endogenous protein synthesis in a rabbit reticulocyte cell-free translation system. With highly purified hybridons inhibition was completely specific for beta globin. Mixtures of two oligonucleotides binding contiguously to the mRNA were more effective than either oligomer alone.
Maher and Dolnick, Nucleic Acids Res. 16: 3341-3358 (1988) report that antisense oligonucleotides containing either anionic diester or neutral methylphosphonate internucleoside linkages were prepared by automated synthesis, and subsequently compared for their ability to arrest translation of human dihydrofolate reductase (DHFR) mRNA in a nuclease treated rabbit reticulocyte lysate. In the case of oligodeoxyribonucleotides, tandem targeting of three 14-mers resulted in synergistic and complete selective inhibition of DHFR synthesis at a total oligomer concentration of 25 .mu.M.
Kutyavin et al, FEBS Lett. 238: 35-38 (1988) report that mono- and diphenazinium derivatives of oligonucleotides complementary to the DNA sequence adjacent to the target sequence of the addressed alkylation of DNA significantly enhance the extent and specificity of alkylation by p-(N-2-chloroethyl-N-methylamino(benzylamido) derivatives of the addressing oligonucleotides.